1. Field of the Invention
The present invention relates to image processing, and, in particular, to video compression.
2. Description of the Related Art
Video compression is employed to reduce the bandwidth required for transmission or storage. Many standards have evolved for video compression, such as H.261, H.263, H.263+, and the MPEG-1, 2, and 4 standards. These standards use motion compensation and predictive coding where some frames are predicted from reference frames in order to achieve coding efficiency. They also use variable-length codes (VLCs) for the same purpose. While these techniques are excellent from the point of view of compression, in the presence of channel errors, they can cause propagation of errors over a large part of the sequence.
Many tools have been developed to improve the error resilience of compressed video bit streams, such as resynchronization (resync) markers, data partitioning, and reversible VLCs, which are now are part of the MPEG-4 standard.
When channel errors cause the decoder to lose synchronization of a compressed video bitstream that was encoded using VLCs, all the following data up to the next resyne point in the bitstream will be lost. In the normal encoding mode, this resync point will be the start of the next picture. The use of resync markers splits each picture into video packets by explicitly introducing markers in the bit stream and ensuring that there are no dependencies across the packets. Thus, an error in a packet is confined within that packet.
Data partitioning splits the data according to importance. For example, in motion-compensated predictive coding, the motion is usually more important than the residual (i.e., the inter-frame differences after motion compensation) in terms of importance for the perceived quality. If the motion data are placed earlier in the data packet than the residual data, then a channel error that occurs during transmission of the residual data will not affect the more-important motion data. This further increases the resilience of the bitstream in the presence of errors. Reversible VLCs provide additional localization of bit errors.
In all the coding standards, there are intra-coded pictures (I frames) and predictive-coded pictures (P and B frames). In P and B frames, individual macroblocks can be coded in the intra mode, i.e., without dependence on previously decoded information. Intra-coded pictures and macroblocks are excellent from the point of view of error resilience since they avoid the propagation of errors. However, their compression efficiency is very low. Also, in low-delay applications such as video-phone and video-conferencing, intra frames may result in a large frame skip, following which the motion-compensated prediction will not be very effective. Also, the mean frame rate would drop and the motion will become very jerky.